1. Field of the Invention
The present invention relates to a developer carrier including a developing roller for developing an electrostatic latent image on an image carrier such, as a photoconductive member, a developing device using the developer carrier, a process cartridge comprising the developing device, and a copier, facsimile apparatus, printer, plotter or similar image forming apparatus including these developing device or the process cartridge.
2. Description of the Background Art
In an image forming apparatus of an electrophotographic or electrostatic recording system including a copier, facsimile apparatus, printer or plotter, in general, an electrostatic latent image corresponding to image data is formed on an image, such as a drum-like or belt-like photoconductive member; the latent image on the image carrier is developed by a developer in a developing device as a toner image; the toner image is directly transferred to a sheet as a recording material, or transferred to the sheet via an intermediate transfer member; and the transferred image is fixed on the sheet by a fixing device. As a developing system in such an image forming apparatus, recently, a magnet brush developing system is widely employed. This system uses a two-component developer made up of toner and magnetic carrier in order to improve image of transferring property, reproducibility of the halftone, the stability of the developing characteristics against temperature and humidity.
A developer carrier including the developing roller used for the developing device of the magnetic brush developing system comprises a cylindrically formed developing sleeve and a magnetic body or a magnetic roller including magnets, disposed within the developing sleeve to form a magnetic field for generating the rise of the developer on the surface of the developing sleeve. The magnetic carrier of the developer rises on the developing sleeve along the line of magnetic force generated by the magnetic roller, and a charged toner adheres to the raised magnetic carrier. The magnets of the magnetic roller for forming a plurality of magnetic poles are formed into a rod-like shape. A pole for development, i.e., a main pole of development for raising the developer, is provided at an area corresponding to the developing region on the surface of the developing sleeve, namely the range where the magnet brush rises on the developer carrier and in contact with the image carrier. Movement of at least one of the developing sleeve and magnetic roller causes the developer rising on the surface of the developing sleeve to move toward the developing region.
The developer transported to the developing region rises along the line of magnetic force emitted from the above-mentioned pole for development and a chain-like raised developer deflectingly comes into contact with the surface of the image carrier. Then, the chain-like developer rubs the latent image on the image carrier on the basis of the relative linear speed difference with the image carrier so that the toner in the developer develops the latent image to make it a toner image.
In such a magnet brush developing system using a two-component developer since the linear speed of the developing sleeve for transporting the developer is to be set faster than the linear speed of the image carrier, a phenomenon that the rear end part of the image becomes pale or a phenomenon of the omission of the trailing edge of the image occurs. This is caused by the fact that the development is delayed in relation to the change of the latent image because the movement of the toner in the developer toward the thickness direction of the developer in the developing region takes time.
As disclosed in Japanese Patent Laid-Open Publication No. 2001-27849, in a developing device of the magnet brush developing system using a two-component developer, phenomena such as omission of the trailing edge of an image, thinning of a line or un-uniformity of the dots can be avoided by shortening a development gap while narrowing a nip for development, and by forming a uniform, short and dense magnet brush without lowering the developing capacity, uniformity, and contamination of the background. Actually, the density of the magnet brush is heightened and the development gap is shortened by narrowing a width of the nip for development and generating the uniform developing electric field. As a result, the moving time of the toner of the magnet brush from the image carrier side to the developer carrier side is reduced when the magnet brush rubs the non-image area on the image carrier in the developing region. Further, a narrow width of the nip for development is obtained by narrowing the width of the pole for development of the magnet in the developing sleeve, and thinning the rising region of the developer. The publication, in addition, proposes a construction with 40% or more of an attenuation ratio of a flux density in a normal direction of the pole for developing of the magnet roller, a nip width of 2 mm or less, and the development gap of 400 μm or less.
The developing system forming a uniform, short and dense magnet brush with narrowed width of the nip and shortened development gap is referred to as an SLIC (Sharp Line Contact magnetic brush development), and the developing device using this developing system is referred to as an SLIC developing device.
In this SLIC developing device, a developing roller as developer carrier has, for example, an attenuation ratio of 40% or more of a flux density in a normal direction (hereinafter referred to as a flux density) of the pole for development, preferably 50% or more. For attaining this attenuation ratio, the pole for development composed with a half-value width of 22° or lower, preferably 21° or lower is used. The half-value width means an angle width indicating a half value of the maximum normal magnet force of the magnetic force distribution curve in the normal direction or the peak flux.
In the SLIC developing device, such a rise of short and dense magnetic brush can be obtained by using such a developing roller so that the width of the nip for development can be narrowed, the movement of the toner to the image carrier can be suppressed, and the lowering of the developing capacity due to the narrow width of the nip for development can be avoided by the dense developing brush.
However, the following problems occur in the developing roller mentioned above:
(1) A proper half-value width varies with the outer diameter of the developing roller.
(2) A difference occurs in the image quality rank, even with the same half-value width. Or, even if the half-value width is narrow, the image quality is degraded from that of a wide roller case.
The above problem (1) is considered to be caused by the fact that the larger the outer diameter of the developing roller is, the wider the width of the nip for development is, with the same half-value width. As for the problem (2), the developing roller normally rotates with a peripheral speed about 1.5 to 2.5 times of that of the image carrier. Therefore, the development of the electrostatic image electrically formed on the latent image carrier, is started at the upstream side of the contact point with the magnet brush.
Since the developer in the magnet brush rubs over the toner once developed, the contribution of the state of the magnet brush at the downstream side of the contact-completion point of the magnet brush with the latent image carrier is considered to be large.